1. Field of the Invention
The present invention relates to a photosensitive lithographic printing plate which is sensitive to a laser light having a wavelength ranging from 390 to 430 nm and which is suitable for direct drawing from digital signals of e.g. computers, and a method for making a printing plate.
2. Discussion of the Background
Heretofore, a lithography method has been widely used for the microfabrication of e.g. lithographic printing plates, printed boards, color filters, large scale integrated circuits (LSI), thin film transistors (TFT), liquid crystal displays (LCD), plasma display panels (PDP) and semiconductor packagings (TAB), wherein an image forming material having a layer of a photosensitive composition formed on the surface of a support is image-exposed through a mask, to form a pattern which utilizes the difference in solubility of a developer between exposed portions and non-exposed portions of the photosensitive layer.
For example, a photopolymerization initiation system for the resist material for color filters, is a combination of a hexaarylbiimidazole derivative with an aminobenzophenone derivative as taught, for example, in JP 11-327127.
However, the resist material for color filters disclosed in JP 11-327127 is not intended to be exposed to laser light from 390 to 430 nm, but rather is subjected through a mask to the light from the like of e.g. a high-pressure mercury-vapor lamp. Further, since the material contains a large amount of a pigment, a large quantity of energy of 200 mJ/cm2, for example, is required for image formation.
Further, U.S. Pat. No. 5,863,678 discloses a resist material for color filters containing a titanocene compound and a dialkylaminobenzene compound as a photopolymerization initiation system. This patent discloses the use of various kinds of light sources. However, the resist material is also exposed to light through a mask from a high pressure mercury vapor lamp source, practically, and the exposure energy of the light is high at 70 or more mJ/cm2 
Further, the above patent does not disclose safe light properties under a yellow lamp for a photosensitive lithographic printing plate.
On the other hand, the laser direct drawing method has attracted attention in recent years as a technique of directly forming an image from digital information supplied by a computer, without using a mask, by employing a laser light as a light source for exposure, since improvement in not only productivity but resolution and accuracy of position and the like is desired. Accordingly, the utilization of a laser light in the lithography method has been actively studied.
With respect to the laser light, various light sources emitting light from the ultraviolet to infrared region are known. Potential laser light for use in image exposure, is light in the visible to infrared region emitted by devices, such as an argon ion laser, a helium-neon laser, a YAG laser and a semiconductor laser. These devices are mentioned from the viewpoint of output, stability, photoperceptivity, cost and the like. For example, various photosensitive compositions have been proposed for use with an argon ion laser having a wavelength of 488 nm and a FD-YAG laser having a wavelength of 532 nm, and a photosensitive lithographic printing plate for exposure to light from these lasers has been placed into practical use. Examples of such photosensitive compositions are a combination of a titanocene compound with a bipyromethene complex and a combination of titanocene with a coumarin derivative as described, for example, in JP 11-271969.
However, no conventional compositions are known which are responsive upon exposure to a violaceous laser light having a wavelength of from 390 to 430 nm.
Further, in an image formation method utilizing such visible laser light, a photosensitive composition showing an adequate absorption at the visible region is used. However, the sensitivity of the composition to violaceous laser light having a wavelength ranging from 390 to 430 nm tends to be inadequate, and safe light properties under a yellow lamp (under environment containing a light having a wavelength of from about 500 to about 750 nm) tend to be poor in some cases, and accordingly the operation has to be conducted in a dark room environment using a red lamp.
On the other hand, JP 61-117549 discloses, as a photosensitive lithographic printing plate which has excellent safe light properties, a photosensitive lithographic printing plate having a protective layer containing a specific coloring agent formed on a photopolymerizable photosensitive composition layer. It discloses, as an exposure source, general-purpose light sources such as a high-pressure mercury-vapor lamps and a metal halide lamps, argon ion lasers, helium-cadmium lasers and krypton lasers as well, and it discloses as a polymerization initiator a complex system of biimidazole and Michler's ketone. However, JP 61-117549, specifically discloses that a composition having a relative sensitivity to light of a wavelength at 480 nm when exposed to a xenon lamp, and a composition responsive to radiation energies required for image formation at 488 nm using an argon ion laser and a composition responsive to light of 442 nm in a the case of using a helium-cadmium laser, have attracted attention The publication discloses that no attention has been paid to compositions responsive to laser light having an oscillation wavelength of from 390 to 430 nm, particularly from 400 to 420 nm. It also discloses that a composition responsive to radiation energy required for image formation of 0.6 to 1.0 mJ/cm2 to light from a helium-cadmium laser having a wavelength of 442 nm not yet sufficient.
On the contrary, along with significant progress in laser technology in recent years, a laser emitting light in the ultraviolet region, which make operation under a bright room environment such as under a yellow lamp, particularly a semiconductor laser which can stably oscillate in a wavelength region of from 390 to 430 nm, particularly from 400 to 420 nm, has been developed. However, no photosensitive lithographic printing plate suitable for exposure to laser light within the violaceous light region (light ranging from 390 to 430 nm in wavelength) has yet been found.